Adds Backus' 1977 ACM Turing Award lecture

Suggestion to add Backus' superb ACM Turing Award lecture from 1977 on "function-level" programming vs "von Neumann" style of programs with tight coupling of semantics, state transitions, inability to provide combinatorial forms etc. One of the best works that sparked research into functional programming.

Migrated re: https://github.com/arc90/papers-we-love/pull/4